How Well Does New Fiberglass Bond To Already Cured Fiberglass?

I have long been under the impression that when bonding new fiberglass to old, the bond is purely mechanical and will not hold up long term. When bonding new components (such as spoilers, ground effects, wheel tubs, etc) to an already cured body, what is the best method for creating a bond that will last long term without cracking or breaking?

Mind you, I've yet to build a car or do major fiberglass repairs. My experience thus far has been limited to duckboat building (where everything was laid up wet), compound bow limb repair (uh, yes I did!), and some other miscellaneous projects. The largest job I've ever done (besides a duckboat) was to repair the bucket on an electrical bucket truck. In that instance, I used epoxy resin with cloth to re-attach a control panel cover to the bucket. I haven't seen that truck in about six years so I hope that repair is still holding up.

Anyway, I would like to hear some thoughts from some much more experienced fiberglass craftsman here.

Re: How Well Does New Fiberglass Bond To Already Cured Fiberglass?

You are correct. It is strictly a mechanical bond. Different fiberglass resins have different windows of how long they are "open" and crosslink to the next layer. Generally speaking the higher strength the shorter the window. I use a ISO-DCPD hybrid and it will generally not cross link to the next layer after 24-30 hours. Some general purpose resins that window is open for a few days.

Anyway you need to determine what type of bond you need. Meaning what is the most likely mode of failure? Tension, compression, twist? I generally use two products. For straight glass to glass like I'm bonding two peices together for a body repair, I use vette bond. It is old school but in 20+ years it has not failed me. Unlike marglass and duraglass, vette bond has greater adhesion strength and finishes better. For applications where I need to bond metal to fiberglass or where I want to the tinyest amount of give like bonding the liner to the back of a fiberglass hood. I use a two part adhesive from FUSOR that is designed for that purpose. If you need to add a layer of fiberglass to a very cured (years) fiberglass. Epoxy is a most because of its adhesive strength versus a polyester or a hybrid. All just my opinion, I'm sure others have different opinions. But definately a mechanical bond, there is no crosslinking after a week for sure.

Re: How Well Does New Fiberglass Bond To Already Cured Fiberglass?

If the area is sanded and prepared properly the bond should not fail. I have repaired tons of "things" and if an accident occurred, the area that failed was not the repaired area. I have been in the automotive field and involved with the Aerospace composite industry for more years than I want to admit to. I have repaired multiple composite and fiberglass components, bodies and "things" that are flown for the space program, and have used a wide array of extremely expensive composite products. For most applications such as this forum is used for, a standard resin such as from Home Depot will have great success. It is fairly inexpensive and can be very easy to use. Just my thoughts.
Dennis

Re: How Well Does New Fiberglass Bond To Already Cured Fiberglass?

Vette Panel Adhesive from Evercoat if you need a quart or a gallon. FUSOR 100 if you just need small amounts ($$$) Although plain resin WILL bond if the surfaces are properly prepared, it will be mostly if not entirely a mechanical bond.

Re: How Well Does New Fiberglass Bond To Already Cured Fiberglass?

Thank you all for your replies! I appreciate it very much.

I bought some 4'x8' fiberglass panels a couple of years ago. They are slightly less than 1/8" thick, smooth on one side and slightly rough on the other. I've been wanting to build a new duck boat with them.

I was also thinking that as flexible as they are, you could probably make a decent car body with them.

Re: How Well Does New Fiberglass Bond To Already Cured Fiberglass?

Something else to consider is that any new resin will continue to shrink over the next year or so while the old work will have stabilized. I did some mods to my car that looked perfect. Had everything block sanded, painted and rubbed out. After about a year with the light just right I can start to see all the mods I made start to show. Not bad but I know where to look.